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From: Cornelia Huck <>
To: Alex Williamson <>
Subject: Re: [PATCH v4 1/2] vfio/mdev: Check globally for duplicate devices
Date: Tue, 22 May 2018 10:13:46 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <20180518191025.3187.29141.stgit@gimli.home>

On Fri, 18 May 2018 13:10:25 -0600
Alex Williamson <> wrote:

> When we create an mdev device, we check for duplicates against the
> parent device and return -EEXIST if found, but the mdev device
> namespace is global since we'll link all devices from the bus.  We do
> catch this later in sysfs_do_create_link_sd() to return -EEXIST, but
> with it comes a kernel warning and stack trace for trying to create
> duplicate sysfs links, which makes it an undesirable response.
> Therefore we should really be looking for duplicates across all mdev
> parent devices, or as implemented here, against our mdev device list.
> Using mdev_list to prevent duplicates means that we can remove
> mdev_parent.lock, but in order not to serialize mdev device creation
> and removal globally, we add which allows UUIDs to
> be reserved such that we can drop the mdev_list_lock before the mdev
> device is fully in place.
> Two behavioral notes; first, mdev_parent.lock had the side-effect of
> serializing mdev create and remove ops per parent device.  This was
> an implementation detail, not an intentional guarantee provided to
> the mdev vendor drivers.  Vendor drivers can trivially provide this
> serialization internally if necessary.  Second, review comments note
> the new -EAGAIN behavior when the device, and in particular the remove
> attribute, becomes visible in sysfs.  If a remove is triggered prior
> to completion of mdev_device_create() the user will see a -EAGAIN
> error.  While the errno is different, receiving an error during this
> period is not, the previous implementation returned -ENODEV for the
> same condition.  Furthermore, the consistency to the user is improved
> in the case where mdev_device_remove_ops() returns error.  Previously
> concurrent calls to mdev_device_remove() could see the device
> disappear with -ENODEV and return in the case of error.  Now a user
> would see -EAGAIN while the device is in this transitory state.
> Signed-off-by: Alex Williamson <>
> ---
>  Documentation/vfio-mediated-device.txt |    5 ++
>  drivers/vfio/mdev/mdev_core.c          |  102 +++++++++++---------------------
>  drivers/vfio/mdev/mdev_private.h       |    2 -
>  3 files changed, 42 insertions(+), 67 deletions(-)

Reviewed-by: Cornelia Huck <>

I think it is better to deal with any possible vendor driver
implications on top of this (I still believe that vfio-ccw is fine).

  parent reply	other threads:[~2018-05-22  8:13 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-18 19:10 [PATCH v4 0/2] vfio/mdev: Device namespace protection Alex Williamson
2018-05-18 19:10 ` [PATCH v4 1/2] vfio/mdev: Check globally for duplicate devices Alex Williamson
2018-05-18 19:37   ` Kirti Wankhede
2018-05-22  8:13   ` Cornelia Huck [this message]
2018-05-22 15:53     ` Alex Williamson
2018-05-23  4:53       ` Zhenyu Wang
2018-05-18 19:10 ` [PATCH v4 2/2] vfio/mdev: Re-order sysfs attribute creation Alex Williamson
2018-05-18 19:38   ` Kirti Wankhede
2018-05-22  8:14   ` Cornelia Huck
2018-05-18 19:37 ` [PATCH v4 0/2] vfio/mdev: Device namespace protection Kirti Wankhede
2018-05-22 17:17 ` Halil Pasic
2018-05-22 18:38   ` Alex Williamson
2018-05-23  8:56     ` Cornelia Huck
2018-05-23 12:29       ` Halil Pasic
2018-05-23 13:34         ` Cornelia Huck

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